The mobile communication service has continuously developed from the first generation (1G) mobile communication service mainly for voice communication of a low quality as was provided by an analogue cellular type Advanced Mobile Phone System (hereinafter, referred to as “AMPS”) which started its service late in the 1980's. The second generation (2G) mobile communication service enabled an improved voice communication and a data service at a low speed (14.4 Kbps) by employing a digital cellular type Global System for Mobile (GSM) scheme, a Code Division Multiple Access (CDMA) scheme, a Time Division Multiple Access (CDMA) scheme, etc. In the 2.5th generation (2.5G) mobile communication service, the frequency of GHz band was secured and the Personal Communication Service (PCS) available worldwide was developed, so that the 2.5G mobile communication service achieved an improved voice communication service together with a data communication service at an improved speed (144 Kbps) still low as it is.
Each of the mobile communication networks for the mobile communication services up to the 2.5th generation includes various communication apparatuses such as a user terminal, a base station transmitter, a base station controller, a mobile switching center, a Home Location Register (HLR) and a Visitor Location Register (UR).
The third generation (3G) mobile communication service is provided by two different kinds of systems including a Wideband CDMA (WCDMA) system and a CDMA-2000 system. The WCDMA system is an asynchronous mobile communication system proposed mainly by the 3rd Generation Partnership Project (3GPP) and the CDMA-2000 system is a synchronous mobile communication system proposed mainly by the 3GPP2. Especially for the WCDMA system, many communication service providers all over the world are providing or preparing to provide services by employing wireless protocols recommended by the International Mobile Telecommunications-2000 (IMT-2000).
The WCDMA system achieves a high communication quality and employs the spread spectrum scheme, so it is proper for transmission of even a large quantity of data. The WCDMA communication system uses an Adaptive Multi-Rate (AMR) codec having a transmit rate of 12.2 Kbps˜4.75 Kbps for voice coding and supports a high mobility enough to allow even a user moving at a speed of 100 Km per hour to perform communication. Many nations are now adopting the WCDMA communication scheme, and the 3GPP constructed by organs of many countries including the United States, Europe, Republic of Korea, Japan, the People's Republic of China, etc. is now continuously developing technology specifications for the WCDMA communication.
Recently, due to the advantages of the asynchronous WCDMA system as described above, countries basically employing the synchronous CDMA-2000 network, such as the United States, Republic of Korea and the People's Republic of China, have started to construct a WCDMA network in order to provide a WCDMA service.
As a result, a mobile communication terminal capable of supporting both the synchronous communication network and the asynchronous communication network has become necessary and a Multi-Mode Multi-Band (MMMB) mobile communication terminal has appeared to meet such a necessity.
The Multi-Mode includes both a synchronous mode and an asynchronous mode, and the Multi-Band includes the frequency band of 800 MHz of the 2G mobile communication service, the frequency band of 800 MHz or 1.8 GHz of the 2.5G mobile communication service, the frequency band of about 2 GHz of the 3G mobile communication service, and a frequency band of the 4G mobile communication service to be provided in the future.
A mode switching between the asynchronous mode and the synchronous mode is necessary when the MMMB mobile communication terminal moves into an area of a synchronous communication network from an overlap area between areas of synchronous and asynchronous communication networks. According to the conventional method for mode switching, a modem for call connection with the synchronous communication network is operated after the mobile communication terminal completely escapes from the overlap area and the call connection with the asynchronous communication network is interrupted. Therefore, the mode switching according to the conventional method requires an interruption interval of at least approximately 20 seconds during which the communication is interrupted.